The present invention concerns a permanent magnet powder having excellent magnetic properties, particularly, large maximum energy product and high coercive force, which can be provided with low costs. The invention also concerns the process for preparing the magnet powder, and the resin-bonded magnet with the magnet powder.
Rare earth metal-based isotropic bonded magnets are advantageous in that they have large maximum energy product and that they are easily processed to any desired shapes, and therefore, widely used in various electronic devices and office automation devices, particularly, for parts of small electric motors. Due to the trend of seeking higher performance and reduced sizes of the devices, further improvement in the performance of the bonded magnets used therein has been demanded.
Currently, the majority of the rare earth-based bonded magnets is isotropic bonded magnets prepared with a NdFeB-based magnet powder which is a product of so-called melt-spinning and by bonding the magnet powder with a binder resin. The maximum energy products of these magnets are, as to the compression-molded magnets, in the range of 8-12 MGOe, and as to the injection-molded magnets, in the range of 5-8 MGOe. Isotropic magnets are, though the maximum energy products are lower than those of anisotropic magnets, of high productivity, because no magnetic field is necessary to apply at the processing. Also, it is advantageous that freedom of the magnetizing patterns is high. The rare earth-based bonded magnets have been thus accepted by the market, and the major parts of the rare earth-based bonded magnets are made with isotropic magnet materials.
Recently, as a magnet material to which the same performance as, or even higher than that of NdFeB magnet alloys can be expected, SmFeN magnet alloys have been attracting public attention. For example, Japanese Patent No.2703218 and also U.S. Pat. No. 5,186,766 corresponding thereto disclose an SmFeN anisotropic magnet material having a Th2Zn17 type crystal structure. Because this material is anisotropic it is necessary to apply magnetic field at the processing and therefore, the process for production is more troublesome than in the case of producing the isotropic magnets.
In regard to the isotropic SmFeN magnet materials a research report was published in J. Appl. Phys. 70, 6 (1991), p.3188-3196. The literature reported that the crystal structures of Sm-Fe powder prepared by the melt-spinning the molten alloy and the crystal structure of SmFeN prepared by the subsequent nitriding of the powder depended on the alloy composition and the conditions of quenching, and that the crystal structure was Th2Zn17 type or TbCu7 type.
The literature disclosed magnetic properties of the magnet powders which were prepared by quenching at peripheral speed of the quenching roll of 10 m/sec., 50 m/sec. or 60 m/sec. followed by nitriding the obtained powder. The best magnetic properties were achieved at the peripheral speed of 60 m/sec., which are as follows:
Th2Zn17 type16.7kA/cm (21.0 kOe)65.6kJ/m3 (8.24 MGOe)TbCu7 type4.9kA/cm (6.2 kOe)69.6kJ/m3 (8.75 MGOe)
The latter type of crystal structure, TbCu7 type, has insufficient coercive force for practical magnets. The authors said in the literature that, in order to achieve high hard magnetic properties, it is necessary to decrease the TbCu7 structure to the extent as small as possible. In other words, the report suggested choice of Th2Zn17 type. Though this structure, Th2Zn17 type, gives sufficient coercive force, the maximum energy product obtained is at best 8 MGOe or so. It was thus concluded that, in view of the fact that the maximum energy product of generally used NdFeB quenched magnet powder is around 15 MGOe, the magnetic material disclosed in this report is not suitable for practical use.
Efforts for improving the performance of the SmFeN quenched magnet powder have been continued after the above report, and resulted in achieving practical magnetic properties. Examples of such publications are “Journal of the Japan Society of Powder and Powder Metallurgy” 46, 6 (1999) p.581-588 and the U.S. Pat. No. 5,750,044. Isotropic, bonded magnets based on SmZrFeCoN disclosed in these literatures exhibit magnetic properties better than those of the magnets with the SmFeN quenched powder described in the above literature, and the performance is near that of NdFeB bonded magnets.
However, as seen from the graphs in the above United States Patent, it is necessary to carry out such high rate quenching as the peripheral speed of quenching roll of 50-100 m/sec. This peripheral speed range is about several times of that used in the conventional production of NdFeB magnetic powder, and the quenching will at first suffer from mechanical problems. Even though such problems could be overcome, there will be caused further difficulties in production of the magnet powder itself, such as decreased yield of quenched ribbon at the quenching step or lowered quantity of powder due to insufficiently quenched powder. It is also a problem in SmZrFeCoN magnet that considerable amount of Zr, which is relatively expensive, is used.
We have conducted research to solve the above noted problems in the SmFeN magnet materials and to develop an SmFeN isotropic magnet material with reduced costs. We have discovered that employment of the producing conditions different from those of the known producing methods will eliminate use of expensive materials and necessity of extremely high rate quenching, and thus, established the powder magnet material which has advantages in industrial practice.